Superabsorbents are water-insoluble, crosslinked polymers capable of imbibing and retaining, under pressure, large amounts of aqueous fluids, especially bodily fluids, preferably urine or blood, by swelling and forming hydrogels. In general, these imbibitions of fluid amount to not less than 10 times or even not less than 100 times the dry weight of the superabsorbents, or superabsorbent compositions, for water. Owing to these characteristic properties, these polymers have their main application in the absorbent cores of hygiene articles such as baby diapers, incontinence products or sanitary napkins. A comprehensive overview of superabsorbents and superabsorbent compositions, their use and their production is provided by F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology”, Wiley-VCH, New York, 1998.
Superabsorbents are generally prepared by free-radical polymerization of acid-functional, usually partially neutralized monomers in the presence of crosslinkers. By varying the monomer composition, the crosslinkers and also the polymerization conditions and the processing conditions for the hydrogel obtained after polymerization, polymers having different absorption properties can be prepared. Further possibilities are offered by the production of graft polymers for example by using chemically modified starch, cellulose and polyvinyl alcohol as described in DE-A-26 12 846.
The current trend in diaper construction in particular is to produce ever thinner absorbent cores having a reduced cellulose fiber content and an increased superabsorbent content. The advantage of thinner designs manifests itself not only in an improved wearing comfort, but also in reduced costs for packaging and stockholding. The latest generation of absorbent cores, which is described for example in WO-A-2008/155722, WO-A-2008/155711, WO-A-2008/155710, WO-A-2008/155702, WO-A-2008/155701, WO-A-2008/155699, EP-A-1 225 857, WO-A-01/15647, WO-A-2011/120504, DE-A-10 2009 049 450, WO-A-2008/117109, WO-A-97/11659, EP-A-0 826 349, WO-A-98/37846, WO-A-95/11653, WO-A-95/11651, WO-A-95/11652, WO-A-95/11654, WO-A-2004/071363 or WO-A-01/89439, is essentially cellulose-free (and hence the diapers of this type are also referred to as fluffless diapers). Superabsorbent particle immobilization, effected in cellulose-containing absorbent cores by the cellulose fibers, can be achieved in this latest generation of absorbent cores by, for example, thermoplastic fibers immobilizing the superabsorbent particles on a substrate surface.
The trend towards ever thinner diaper designs and the disappearance of the temporary fluid-storing and conducting function of cellulose fibers has resulted in a distinct change in the performance profile required of superabsorbents. What is of decisive importance now is the ability of the hydrogel to prevent leakage of urine directly in the course of micturation. This is achieved by the property of the superabsorbent/hydrogel whereby the fluid is sufficiently imbibed, and dispersed in the gel layer, during swelling while at the same time the amount of unbound urine in the diaper is minimized. Advantageous superabsorbents also have good transfer properties and so ensure optimum utilization of the entire hygiene article.
However, prior art superabsorbents are insufficiently suitable for use in the above-described new generation of cellulose-free diaper designs.
The problem addressed by the present invention was therefore that of overcoming the superabsorbent-related disadvantages that surrender to the prior art.
More particularly, the problem addressed by the present invention was that of providing superabsorbents that are particularly advantageous for use in absorbent cores having a low cellulose fiber content, for example in the cellulose-free designs described in WO-A-2008/155722, WO-A-2008/155711, WO-A-2008/155710, WO-A-2008/155702, WO-A-2008/155701 or WO-A-2008/155699.
Another problem addressed by the present invention was that of providing a selection process for selecting, from the multiplicity of currently known superabsorbent materials, those superabsorbents capable of being reliably used in the cellulose-free designs described in WO-A-2008/155722, WO-A-2008/155711, WO-A-2008/155710, WO-A-2008/155702, WO-A-2008/155701 or WO-A-2008/155699 for example.